The evolution to more advanced satellite systems, in particular the high throughput satellite (HTS) type of broadband systems with increasingly smaller spot beams, results in a high probability of mismatch between the satellite bandwidth and power and the actual traffic distribution over the beams in the service area. The multi-port amplifier (MPA) is an integral part of the satellite communication payload that will be fundamental in providing the flexible power allocation capability to satisfy the need to flexibly allocate power across beams, primarily to compensate for weather conditions and variations in traffic loading for each beam.
An MPA generally comprises an input hybrid matrix (IHM) which divides signals, an output hybrid matrix (OHM) which combines signals, and a plurality of high power amplifiers (HPAs)—either Solid State Power Amplifiers (SSPAs) or more traditional Travelling Wave Tube Amplifiers (TWTAs). The MPA components are arranged and aligned such that a signal input to an arbitrary port is divided by the IHM, then amplified by the HPAs, and finally recombined coherently by the OHM at a specific output port. The MPA provides access for each input port equally to each amplifier, and equal gain for all input signals. The MPA output power of each signal is proportional to its input power, providing output power flexibility by controlling the input power.
Such technology has been essential for narrowband mobile-satellite service (MSS) satellite systems at the lower L and S frequency bands, but its application at the higher Ku and Ka frequency bands is much more difficult due to the increasing difficulty of coherent power combining at the shorter wavelengths and larger bandwidths. The challenges of phase and amplitude misalignment of individual amplifiers of an MPA at the Ku/Ka-bands, and hence that of isolation and signal combining performance, become considerably greater and introduce complications regarding the feasibility of operating MPAs at these frequencies onboard a satellite and over the required service life. To achieve coherent power combining and minimize the port-to-port leakages, an in-orbit calibration system is critical for a high performance MPA. In general, the MPA calibration system will comprise a subsystem to detect/measure and compensate for the phase and amplitude errors introduced between the input and output of the MPA.
A few calibration methods for maintaining MPA isolation performance have been invented recently. One type of the calibration methods is to measure the gain/phase of the individual amplifiers and maintain the gain/phase tracking [U.S. Pat. Nos. 7,558,541, 7,965,136]. A specific form of frequency/phase modulated calibration signals are injected to the inputs of the individual amplifiers and the gain and phase errors of individual amplifiers are detected in baseband. However, the calibration accuracy is limited by the tolerance of the gain and phase detection and the imperfection of passive components which are not within the calibration loop.
Another type of calibration method is the phase and/or gain matching of individual amplifier pairs by detecting and minimizing the null level [U.S. Pat. Nos. 7,088,173, 8,103,225]. The test signals are injected using couplers within the IHM, or detected using couplers within the OHM, affecting the IHM/OHM performance. The calibration signals are in simple form, however, the calibration accuracy is limited by the hybrid imperfection and multi-loop calibration.
A third type of calibration method relies on detection at the MPA outputs, and estimate/adjust the composite signals at the individual amplifier outputs or the complex gains of the individual amplifiers with matrix manipulations [U.S. Pat. Nos. 8,581,663, 9,319,000]. A priori knowledge of unit transfer matrices is needed, and the calibration accuracy is limited by the measurement tolerances and passive component imperfections.
A fourth type of calibration method involves estimation of the special characteristics of the MPA output signals such as probability density function [U.S. Pat. No. 7,822,147], power spectrum [U.S. Pat. No. 8,463,204], or normalized mean cross-correlation [U.S. Pat. No. 9,413,306]. There are possibilities to use the traffic signals in lieu of test signals; however, there are certain constraints on input signal statistics and/or frequency plan. The calibration accuracy is limited by estimating the delta statistics of the output signals in the presence of intermodulation noise.
A fifth type of calibration method involves calibration signal detection on the ground [U.S. Pat. No. 7,965,136; Ian Morris, et al, “Airbus Defence and Space: Ku Band Multiport Amplifier powers HTS Payloads into the future”, AIAA 2015-4340]. However, such methods are constrained by the location of ground receivers and signal fading issues in propagation.
All above inventions do not provide direct onboard monitoring of MPA port-to-port leakages, and the calibration accuracy is limited by the measurement tolerances, passive component imperfection and/or intermodulation noises. Therefore, it would be desirable to have a calibration system that monitors the MPA leakages onboard the satellite, and directly minimize the MPA leakages. It could also be desirable to compromise between onboard and on-ground hardware/software requirements.
Accordingly, there is a need for an improved calibration system and method for optimizing leakage performance of a multi-port amplifier.